I have a mixed Exchange 2007 and Exchange 2003 environment. In the environment I have :-

* An Exchange 2003 mailbox server

* An Exchange 2003 front-end server

* An Exchange 2007 CAS

* An Exchange 2007 mailbox server

The 2007 CAS runs Exchange 2007 SP 1. From a client machine accessing the Exchange 2007 mailboxes using Outlook Web Access access to public folders is working. When I click on a Public Folder which is hosted on the Exchange 2003 world, then I get some text in the right hand page saying :-

Click here to open this folder with Microsoft Exchange 2003 Outlook Web Access

When I click that link I was getting a page saying : 501 & 505…

HTTP 501 Not Implemented/HTTP 505 Version Not Supported

Further if I go to https://cas/public, I actually get redirected to https://cas/owa.

The issue is “resolved” by updating the mailbox server to Exchange 2007 SP 1.